1. Field of the Invention
The present invention relates to a process for preparing a superconductor. More specifically, the present invention relates to a process for preparing a superconductor of Bi-(Pb)-Sr-Ca-Cu-0 system or Tl-Ba-Ca-Cu-0 system in a shorter firing time.
2. Description of the Related Art
High temperature superconductive materials exhibiting superconductivity at a temperature of liquid nitrogen are under intensive investigation and the development of the utilization of these materials is proceeding rapidly. Particularly, high temperature superconductive materials exhibiting superconductivity at above 100 K are under investigation and development, since they allow a wide temperature margin for liquid nitrogen application, theoretically high critical current density Jc and critical magnetic field Hc, and are expected to provide superconductive materials with a higher Tc which can be utilized as thin films and wiring.
Y-Ba-Cu-0 system superconductive materials have a high critical temperature of about 90 K but are chemically unstable, resulting in a loss of the superconductivity by a reaction with H.sub.2 O, CO.sub.2, etc., when placed in air, which causes problems in the utilization thereof.
As superconductive materials having a critical temperature higher than 100 K, Bi-Sr-Ca-Cu-0 system and Tl-Ba-Ca-Cu-0 system superconductor materials are known (J.J.A.P. vol. 27, No. 2, 1988, pp 209-210; NIKKEI ELECTRONICS 1988.4.18 (No. 55) pp 175-177). These Bi-system and Tl-system superconductive materials are not susceptible to water and oxygen, are easily made and processed, and are not source-limited since they do not contain a rare earth element. Nevertheless, the Bi-system superconductive materials have a low Tc phase of about 80 K and a high Tc phase of about 110k, and the Tl-system superconductive materials have a low Tc phase of about 105 K and a high Tc phase of about 125 K: Note, the high Tc phases of these materials are not formed individually.
It is known in the prior art that the proportion of the high Tc phase of the Bi-Sr-Ca-Cu-0 system superconductive material can be increased by adding PbO (NIKKEI SUPERCONDUCTOR 1988.5.16, pp 2-3). In this process, a part of Bi is replaced by Pb, salts of oxalates in a predetermined ratio (Bi:Pb:Sr:Ca:Cu:0 =0.7:0.3:1.0:1.0:1.8) are used as starting materials, and calcination at 800.degree. C for 12 hours and firing at 845.degree. C. for 240 hours are carried out. At the calcination stage, the formation of the low Tc phase must be avoided and intermediate products of Ca.sub.2 PbO.sub.x(x=3 or 4), Ca.sub.x Sr.sub.3-x Cu.sub.5 O.sub.y, Bi.sub.2 Sr.sub.2 Cu.sub.1 O.sub.x produced, and firing is conducted at 845.degree. C. while the temperature is kept at not higher than 880.degree. C. It is disclosed that almost all of the Pb is lost during the firing; Pb creates a reaction route when forming the high Tc phase and stabilize the formed high Tc phase, but this process disadvantageously requires a long firing time of 240 hours.
Also it was reported by researchers of Kyoto University, Mie University, and Toda Kogyo that a Bi-system bulk with a high Tc phase was obtained by adding Pb, and by researchers of Osaka University and Daikin that an almost single-phased sample is attained by adding Pb to the Bi system. Samples are prepared by two methods, a codecomposition of nitrates and a solid reaction, both of which produce a superconductive material having a high Tc phase of 107.5 K, and a composition ratio of Bi:Pb:Sr:Ca:Cu =0.8:0.2:0.8:1.0:1.4. This process utilizes firing in a low pressure oxygen atmosphere, by which the melting point of the material is lowered and the temperature range for the formation of the high Tc phase is broadened. The firing is conducted at 842.degree. C for 80 hours (solid reaction method) and at 828.degree. C for 36 hours. In both methods, variation of the oxygen pressure allows firing at 800.degree.-820.degree. C., and it is disclosed that, in the nitrate codecomposition method, firing at 828.degree. C for 36 hours produces the high Tc phase at a higher rate and is an example of a short time firing (Trigger 88-8, p 91-92, 1988.7.8).
A Bi-Sr-Cu-0 system superconductive material having a Tc of 7 K was found by Akimitsu et al (Jpn. J. Appl. Phys. 26, 1987, L2080) or Michel (et al Z. Phys. B68 (1987) 421).
Maeda et al found a high Tc phase Bi-Sr-Ca-Cu-0 system superconductive material (Jpn. J. Appl. Phys. 27, 1988, L209), having three superconductive phases with a Tc of 7 K corresponding to n =1, a Tc of 80 K corresponding to n =2 and a Tc of 105 K corresponding to n=3, in Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x. With increase of n from 1 to 3, the number of the CuO layers in the crystal structure is increased and the c axis is varied from 24 A.ANG. to 30 A.ANG. to 37 A.ANG..
Since the single high Tc phases of the above superconductive materials are not synthesized, an investigation of the high Tc phases has not been reported.
Takano et al reported that the proportion of the high Tc phase of the Bi-system superconductive material was increased by substituting Pb for Bi (Jpn. J. Appl. Physics 27, 1988, L1041), but the role of Pb was not clarified.
The purpose of the present invention is to provide a process for preparing a superconductive material with an increased proportion of a high Tc phase in a shorter firing time in comparison with the prior art method of adding PbO.